In which one of the following transition metal complexes, does the metal exhibit zero oxidation state? (a) \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) (b) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{X}_{3}\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}\) (d) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{SO}_{4}\)

Coordination compounds, also known as coordination complexes, are molecules that feature a central metal atom or ion bonded to one or more ligands—molecules or ions that can donate a pair of electrons. The metal in a coordination complex is typically a transition metal. These ligands are attached to the metal center by coordinate covalent bonds, where both electrons in the bond come from the same atom, the ligand. For instance, in the complex \[\mathrm{Ni}(\mathrm{CO})_{4}\], nickel acts as the central metal atom surrounded by carbonyl (\(\mathrm{CO}\)) ligands.

The number of ligands attached to a metal ion is defined by its coordination number, which can vary depending on the size, charge, and electron configuration of the metal ion. An important aspect of coordination compounds is the oxidation state of the metal, which can offer insight into the compound's reactivity and properties. In the exercise, finding the metal with a zero oxidation state involves analyzing the coordination compound’s components, like the nature of its ligands and the overall charge of the complex.

The concept of ligands and their charges is vital to understand coordination chemistry. Ligands are species that have available pairs of electrons for bonding and can be neutral molecules like water (\(\mathrm{H}_2\mathrm{O}\)) and ammonia (\(\mathrm{NH}_3\)) or charged ions such as sulfate (\(\mathrm{SO}_4^{2-}\)). The charge of a ligand is crucial when calculating the oxidation state of the metal in a complex.

In coordination complexes, the charge of the ligands and any potential counter ions will collectively define the overall charge of the complex. If the ligand is neutral, it won’t contribute to the charge of the overall complex, but charged ligands will contribute negatively or positively. For example, the neutral ligand \(\mathrm{CO}\) in \[\mathrm{Ni}(\mathrm{CO})_{4}\] does not add any charge to the complex. Understanding how to identify and count the charge of ligands is fundamental for students in assigning the correct oxidation state to the metal center.

Transition metals occupy the d-block of the periodic table and are characterized by their ability to form a wide variety of chemical compounds, including coordination complexes. One of their distinctive features is the incomplete filling of their d orbitals, which allows them to exhibit multiple oxidation states. An oxidation state is essentially the hypothetical charge that an atom would have if all its bonds to atoms of different elements were 100% ionic.

Having a range of oxidation states allows transition metals to be extremely versatile in forming compounds. For instance, iron can exist in the \(\mathrm{Fe}^{2+}\) and \(\mathrm{Fe}^{3+}\) oxidation states, leading to different properties and reactivity. Coordination compounds can be complex, and the oxidation state of the central metal is key in determining their geometry, magnetism, color, and reactivity. In the exercise, identifying the complex with a metal in the zero oxidation state (\(\mathrm{Ni}(\mathrm{CO})_{4}\)) is based on the principles of oxidation states in transition metals.